Muhammad T. Sajjad

Advance in Novel Boron Nitride Nanosheets to Nanoelectronic Device Applications

We report low-temperature synthesis of large-scale boron nitride nanosheets (BNNSs) and their applications for high-performance Schottky diode and gas sensor. Ten minutes of synthesis with a short-pulse-laser-produced plasma deposition technique yields a large amount of highly flat, transparent BNNSs. A basic reason for using short-pulse plasma beams is to avoid nanosheet thermal ablation or have low heat generated. Consequently, it greatly reduces the stress and yield large, flat BNNSs. The average size of obtained BNNS is around 10 μm and thickness is around 1.7 nm. Carbon element has been used for doping BNNSs and achieving BNNSs-based Schottky diode and gas sensing device. Typical current versus voltage characteristics of diode are examined. The breakdown reverse voltage is around -70 V. This probably indicates that the breakdown electric field of BNNSs-based diode is up to 1 × 10(8) V/cm. Sensing behavior of BNNSs-based gas sensor toward methane diluted with dry air is also characterized. The response time and recovery time are around 3 and 5 s at the operating temperature of 150 °C. Relatively, the sensor has poor sensitivity to oxygen gas.

Mercaptophosphonic acids as efficient linkers in quantum dot sensitized solar cells

Control over the deposition of quantum dots (QDs) on nanostructured semiconductors is very important for the photovoltaic performance of QD sensitized solar cells. The best control is typically achieved using bifunctional molecular linkers, such as mercaptopropionic acid (MPA), to attach the QDs to metal oxides in a specific manner; however some materials, such as ZnO, are not compatible with these molecules due to their pH sensitivity. We have developed new linkers, mercaptophosphonic acids of different length, which allow efficient functionalization of ZnO nanowires and also mesoporous TiO2 without damaging their surface. Detailed XPS and contact angle studies of the mechanism of self-assembly of these acids show that their strong chelation of the oxide surface prevents protonic attack and etching. Using these linkers, we show that colloidal ternary quantum dots, CuInS2, can be conformally and homogeneously deposited on the functionalized metal oxides. Photophysical studies by means of time-resolved photoluminescence spectroscopy confirm efficient electron transfer from the QDs to the metal oxides with the rate and efficiency scaling with respect to the linker length and nature. The efficiency of the QD sensitized solar cells fabricated with such assemblies also strongly depends on the linkers used and follows the trends observed for the charge transfer.

Photophysical and structural characterisation of in situ formed quantum dots

Conjugated polymer-semiconductor quantum dot (QD) composites are attracting increasing attention due to the complementary properties of the two classes of materials. We report a convenient method for in situ formation of QDs, and explore the conditions required for light emission of nanocomposite blends. In particular we explore the properties of nanocomposites of the blue emitting polymer poly[9,9-bis(3,5-di-tert-butylphenyl)-9H-fluorene] together with cadmium sulphide (CdS) and cadmium selenide (CdSe) precursors. We show the formation of emissive quantum dots of CdSe from thermally decomposed precursor. The dots are formed inside the polymer matrix and have a photoluminescence quantum yield of 7.5%. Our results show the importance of appropriate energy level alignment, and are relevant to the application of organic-inorganic systems in optoelectronic devices.

Controlling Exciton Diffusion and Fullerene Distribution in Photovoltaic Blends by Side Chain Modification

The influence of crystallinity on exciton diffusion and fullerene distribution was investigated by blending amorphous and semicrystalline copolymers. We measured exciton diffusion and fluorescence quenching in such blends by dispersing fullerene molecules into them. We find that the diffusion length is more than two times higher in the semicrystalline copolymer than in the amorphous copolymer. We also find that fullerene preferentially mixes into disordered regions of the polymer film. This shows that relatively small differences in molecular structure are important for exciton diffusion and fullerene distribution.

Controlling the emission efficiency of blue-green iridium(III) phosphorescent emitters and applications in solution-processed organic light-emitting diodes

We show that the emission efficiency of blue-green phosphorescent emitters can be controlled through coupling of the excited state to vibrational modes. We controlled this vibrational coupling through choice of different ligands and as a result, complexes with CF3-groups on the ancillary ligand were essentially non-emissive (ΦPL 50%). Emission of the complexes can be drastically improved (30 times higher ΦPL compared to degassed solution for the CF3-containing complexes) by blending them with an inert solid host such as PMMA, which mitigates metal-ligand vibrations. Solution-processed organic light-emitting diodes made from these materials showed efficiency as high as 6.3%.

Fringe structures and tunable bandgap width of 2D boron nitride nanosheets

Summary We report studies of the surface fringe structures and tunable bandgap width of atomic-thin boron nitride nanosheets (BNNSs). BNNSs are synthesized by using digitally controlled pulse deposition techniques. The nanoscale morphologies of BNNSs are characterized by using scanning electron microscope (SEM), and transmission electron microscopy (TEM). In general, the BNNSs appear microscopically flat in the case of low temperature synthesis, whereas at high temperature conditions, it yields various curved structures. Experimental data reveal the evolutions of fringe structures. Functionalization of the BNNSs is completed with hydrogen plasma beam source in order to efficiently control bandgap width. The characterizations are based on Raman scattering spectroscopy, X-ray diffraction (XRD), and FTIR transmittance spectra. Red shifts of spectral lines are clearly visible after the functionalization, indicating the bandgap width of the BNNSs has been changed. However, simple treatments with hydrogen gas do not affect the bandgap width of the BNNSs.

Synthesis and properties of novel star-shaped oligofluorene conjugated systems with BODIPY cores

Summary Star-shaped conjugated systems with varying oligofluorene arm length and substitution patterns of the central BODIPY core have been synthesised, leading to two families of compounds, T-B1–T-B4 and Y-B1–Y-B4, with T- and Y-shaped motifs, respectively. Thermal stability, cyclic voltammetry, absorption and photoluminescence spectroscopy of each member of these two families were studied in order to determine their suitability as emissive materials in photonic applications.

Narrow-band anisotropic electronic structure of ReS2

We have used angle-resolved photoemission spectroscopy to investigate the band structure of

Unprecedented Strong Panchromic Absorption from Proton-Switchable Iridium(III) Azoimidazolate Complexes

{\mathrm{ReS}}_{2}

Correlating photovoltaic properties of a PTB7-Th:PC71BM blend to photophysics and microstructure as a function of thermal annealing

, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of structural distortion and spin-orbit coupling. We further show how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zigzag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone center along